Develop Reference

android.permission.WRITE_SETTINGS what kind of permission Normal? or Dangerous?

I have read Dengerous permission and Normal permission as well. but I didn't find this permission anywhere. I just want to know what kind of permission is it?

WRITE_SETTINGS
added in API level 1
String WRITE_SETTINGS
Allows an application to read or write the system settings.
Note: If the app targets API level 23 or higher, the app user must explicitly grant this permission to the app through a permission management screen. The app requests the user's approval by sending an intent with action ACTION_MANAGE_WRITE_SETTINGS. The app can check whether it has this authorization by calling Settings.System.canWrite().
Protection level: signature
Constant Value: "android.permission.WRITE_SETTINGS"

Special Permissions
There are a couple of permissions that don't behave like normal and dangerous permissions. SYSTEM_ALERT_WINDOW and WRITE_SETTINGS are particularly sensitive, so most apps should not use them. If an app needs one of these permissions, it must declare the permission in the manifest, and send an intent requesting the user's authorization. The system responds to the intent by showing a detailed management screen to the user.
Source: https://developer.android.com/guide/topics/permissions/requesting.html#perm-groups

The protection level is signature. You must ask the user to be granted this permission if your API level is greater than or equal to 23.
You can find the documentation on all permissions here,
and you can find the documentation for the permission you specified here.

Android SYSTEM_ALERT_WINDOW permission

I read that with Android 6.0, users need to manually allow apps to hold this permission by going to app advanced settings and enabling "Draw over other apps". I have a Nexus 5 with Android 6.0 but I don't seem to be prompted to enable this setting. When I install apps from the Play Store that require this permission, such as LastPass, it gets granted automatically.
Why is this so?

It is a new behaviour introduced in Marshmallow 6.0.1.
Every app that requests the SYSTEM_ALERT_WINDOW permission and that is installed through the Play Store (version 6.0.5 or higher is required), will have granted the permission automatically.
If instead the app is sideloaded, the permission is not automatically granted. You can try to download and install the Evernote APK from apkmirror.com. As you can see you need to manually grant the permission in Settings -> Apps -> Draw over other apps.
[The above information is from this post.]
If you want the app to be sideloaded, you show manually show a prompt and direct the user to enable Draw over other apps permissions from the settings. Have a look at Requesting permissions

Every app that requests the SYSTEM_ALERT_WINDOW permission and that is installed through the Play Store (version 6.0.5 or higher is required), will have granted the permission automatically.
Click here! This may help

There are mainly two types of permissions, they are
Normal Permissions
Dangerous Permissions
Normal permissions indicates that there's no great risk to the user's privacy or security in letting apps have those permissions. For example, users would reasonably want to know whether an app can read their contact information, so users have to grant this permission explicitly. By contrast, there's no great risk in allowing an app to vibrate the device, so that permission is designated as normal.
Dangerous permissions cover areas where the app wants data or resources that involve the user's private information, or could potentially affect the user's stored data or the operation of other apps. For example, the ability to read the user's contacts is a dangerous permission. If an app declares that it needs a dangerous permission, the user has to explicitly grant the permission to the app.
In this case, SYSTEM_ALERT_WINDOW comes under Normal permissions, that is if an app declares in its manifest that it needs a normal permission, the system automatically grants the app that permission at install time. The system does not prompt the user to grant normal permissions, and users cannot revoke these permissions.
You can see the list of normal permissions in this link and dangerous permissions here.

Please explain Android permission protection_signature explanation

I got the below explanation from Android documentation, about the new permission model in Android M Preview. Please explain the texts in bold in simple words because I am confused.
If the app requests permissions in the manifest that fall under PROTECTION_SIGNATURE, and the app is signed with the same certificate as the app that declared those permissions, the system grants the requesting app those permissions on installation. Apps cannot request signature permissions at runtime.

Apps can define their own permissions via the manifest. This is referring to those permissions. So if I publish 2 apps, both signed with the same certificate, and app#1 defines a new permission with signature level protection and app#2 uses that permission (by stating so in its manifest) then the system will automatically grant the permission to app#2. Note that this is not new to Android Marshmallow. Only the selective grant/revoke is.
This article will help to explain permissions in general under Android: http://hiqes.com/android-security-part-2

Let me guess.
there are two apps, A and B, they was signed with the same certificate.
first at all, user start using A app, and request permissions EXAMPLE_PERMISSION under PROTECTION_SIGNATURE, then system gives a dialog and tips user that he need to grant it. user click GRANT.
And then, user launch B app, B app wants the same permission, the EXAMPLE_PERMISSION, and when it requests the permission, system auto grant that.
Because of A app has granted it, and A and B have the same certificate.
I guess so.

INTERNET permissions in Android M

Regarding Google's recent announcement about Android M and Permissions model.
Per the official Android documentation:
Limited Permissions Granted at Install Time: When the user installs or
updates the app, the system grants the app all permissions that the
app requests that fall under PROTECTION_NORMAL. For example, alarm
clock and internet permissions fall under PROTECTION_NORMAL, so they
are automatically granted at install time. The system may also grant
the app signature and system permissions, as described in System apps
and signature permissions. The user is not prompted to grant any
permissions at install time.
Particular note that it says:
...the system grants the app all permissions that the app requests.
So, if the app does not have INTERNET permission in its AndroidManifest.xml, it won't be granted access to INTERNET in that case?
Or will an app require to add INTERNET permission in its manifest in order to be able to make network calls?

As for the specific android.permission.INTERNET permission, it is still mandatory for apps that will access the Internet. If a developer were to publish an app without defining it in the Android manifest, an exception will be thrown the first time a connection attempt is made, and the app will possibly crash. This is no different than before.
All that has changed is that there won't be a prompt to the user, the app will still require the permission in the manifest.

Please check this video from Google IO - https://youtu.be/f17qe9vZ8RM?t=18m10s
There is no more Internet permission - the app will have by default access to the internet. Their idea is that if you don't have access to the device data then you can not do anything dangerous
Kind Regards

Regarding Android Permissions and Signature Protection level

I am new to Android and have a question regarding protection level "Signature" for permissions in AndroidManifest.xml.
The Android reference document states about "Signature" protection level:
A permission that the system grants only if the requesting application
is signed with the same certificate as the application that declared
the permission. If the certificates match, the system automatically
grants the permission without notifying the user or asking for the
user's explicit approval.
This implies that permissions which have protection level "Signature" are not available to use by normal application and can only be used Android Dev Team.
What I am wondering about is that how many applications in Android Market or on other sites can have these permissions? Like an application which is used for recording calls has android.permission.DEVICE_POWER in addition to other permissions. Is Android system really granting this permission to this application while installation?
When I tried to use the permission "READ_INPUT_STATE" (new in 2.2) I got the following error in LogCat:
06-28 09:28:34.943: WARN/PackageManager(60): Not granting permission android.permission.READ_INPUT_STATE to package com.example.wheredoyoulive (protectionLevel=2 flags=0x8444)
The same is true for permissions with Protection Level "SignatureOrSystem". There exists a caller application which has CALL_PRIVILEGED permission in addition to other permissions.
Please help me and clear my doubts.
Regards
Abhishek

I believe the purpose of the "Signature" permission level is for two applications by the same developer to be able to share data seamlessly without bothering the user. The READ_INPUT_STATE permission is not intended to be used in applications:
Allows an application to retrieve the current state of keys and switches. This is only for use by the system.
See http://developer.android.com/reference/android/Manifest.permission.html#READ_INPUT_STATE

Facebook home uses this,
once you install it you'll notice that it doesn't request ANY permissions, but explicitly requires that the facebook app be installed, this is so that the system can grant it the necessary permissions by proxy of the Facebook app.
Typically what happens is the Facebook app with advertise facilities for other apps to read your status and news feed, normally these apps would need to explicitly request permission to use them if they are signed under a different certificate or rather private key.